WASHINGTON - Thousands of people gathered in the U.S. capital on Sunday to urge President Barack Obama to honour his recent climate change vows, with TransCanada's proposed Keystone XL pipeline the protest's most popular whipping boy.

Organizers billed the rally, held at the foot of the Washington Monument on the National Mall, as the biggest climate protest in American history.

But despite claims by organizers that 50,000 people were in attendance, the crowd appeared smaller, with one police officer in attendance unofficially pegging it at about 10,000 as it got underway.

Nonetheless, despite the brisk, blustery day, steady streams of people — from a group of nuns to native activists and local high-school students — made their way from surrounding subway stations to spill onto the mall. Many of them waved anti-Keystone signs.

While the rally, called "Forward on Climate," was meant to demand Obama take action not just on Keystone XL but on fracking and coal as well, the pipeline was by far the biggest target. Hundreds of protesters formed a human pipeline and marched to the White House, chanting anti-Keystone slogans as they made their way north.

Several prominent environmentalists addressed the rally, including Bill McKibben, the 350.org founder who has spearheaded the U.S. environmental movement's opposition to Keystone XL.

"This movement's been building a long time," McKibben, who was arrested along with dozens of other activists last week at a White House protest, told the crowd.

"One of the things that's built it is everybody's desire to give the president the support he needs to block this Keystone pipeline."

He branded the pipeline, which would carry bitumen from Alberta's carbon-intensive oilsands to Gulf Coast refineries, as "one of the largest carbon bombs in history."

Michael Brune, the Sierra Club executive director also arrested last week, also focused on the pipeline.

“President Obama holds in his hand a pen and the power to deliver on his promise of hope for our children," he said. "Today, we are asking him to use that pen to reject the Keystone XL tarsands pipeline, and ensure that this dirty, dangerous, export pipeline will never be built.”

Keystone XL has indeed become a flashpoint for McKibben and other high-profile American environmentalists who view it as a symbol of dirty oil, even though the U.S. is already crisscrossed with various pipelines transporting oil through several states.

As the latest protest started in D.C., John Baird, Canada's foreign affairs minister, chided Americans for lecturing Canada on reducing greenhouse gas emissions.

"We adopted the same goals and objectives in terms of climate change … We worked with the Obama administration and harmonized vehicle emission standards, light truck standards," Baird told The Canadian Press in a telephone interview from Lima, Peru.

"We're also taking concrete direct action with respect to dirty, coal-fired electricity generation. Maybe the United States could join Canada on that file."

Baird was responding to U.S. Ambassador David Jacobson, who said last week that Obama's State of the Union address — calling once again for swift action on climate change — should also be interpreted as a challenge to Ottawa.

The mood, evidently, is tense at the moment between the U.S. and Canada on the Keystone file.

Pro-pipeline stakeholders on both sides of the border are increasingly convinced that Obama will demand something in return for approving Keystone by way of fees on greenhouse gas emissions. Such fees would be levied not just on existing American power plants but at the border, making it potentially much more expensive for Canadian oil producers to ship their product stateside.

But one American with ties to Canada, bundled up on Sunday to take part in the rally, says she doesn't think average Canadians will be angry if the U.S. rejects Keystone XL.

Karen Bradley, an American with a house in Musquodoboit Harbour, N.S., says the people in her home away from home are anti-Keystone.

"None of the people in our village are supportive of the pipeline. Pretty much everyone we know up there is upset with Stephen Harper and his government for setting this up — it's a really bad idea," she said.

Bradley added she had no concerns that the Canada-U.S. relationship would suffer if Obama rejects the pipeline.

"It would be damaged in some quarters, but in other quarters it would be celebrated," she said.

"People in our village have seen the effects of climate change, and the effects of man-made stupidity on the climate — there's no more fish, there are incredible weather events — so they have the big picture, even though it's a small village."

Geraldine Thomas-Flurer, co-ordinator of the Yinka Dene Alliance — a coalition of six First Nations from northern British Columbia — echoed Bradley when she said real people, not oil executives or government officials, are suffering due to the oilsands.

"We're here to support our brothers and sisters against Keystone XL, because any expansion of the tarsands would impact everyone," she said.

"There's people that are dying around the tarsands, people that are getting sick, their water is sick, their animals are sick, and they can't live the life that they choose to live. Our government has taken advantage of the people in our communities .... we're tired of being their dirty little secret."

Those both for and against the $7 billion project are anxiously awaiting a decision from the U.S. State Department, which will determine the fate of Keystone XL because it crosses an international border. A decision is expected some time this spring.

Keystone foes have been cheered by the arrival of John Kerry at the State Department. Now Obama's second secretary of state, Kerry was a fierce climate hawk during his 28 years as a Massachusetts senator.

Among those in attendance at Sunday's rally were actresses Rosario Dawson and Evangeline Lilly.

Lilly, a Canadian best known for her role on the TV drama "Lost," apologized to the crowd on behalf of Canada.

“I am ashamed that we are knocking on your door with dirty oil,” she said.

“I want to stand up here as a Canadian and I want to say I am sorry to the workers in Canada and the workers in America who have to go home and look their kids in the eye and know that they are damaging their future."

Busloads of protesters arrived from 28 states for Sunday's rally. A group of people from rural Massachusetts even spent the night at a church in northeast Washington, some of them sleeping on pews, so they could make their way to the rally by subway on Sunday morning.

"We had to be here," said Sally Wainwright, a schoolteacher from the Birkshires area of Massachusetts. "How much longer are we going to continue burning things before we understand the terrible toll it's taking?"

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